1. Field of the Invention
The invention relates to a method for operating a device for delivering a liquid, in particular for the evacuation thereof. The device may be used for example in a motor vehicle in order to deliver a liquid additive to an exhaust-gas treatment device for the purification of the exhaust gases of an internal combustion engine of the motor vehicle, and to feed the liquid additive to the exhaust-gas treatment device in dosed fashion.
2. Related Art
In exhaust-gas treatment devices to which a liquid additive for exhaust-gas purification is fed, the method of selective catalytic reduction (SCR), for example, is implemented. In the method, nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced with the aid of a reducing agent. Ammonia is commonly used as reducing agent. Ammonia is usually stored in motor vehicles not directly but rather in the form of a reducing agent precursor solution. One reducing agent precursor solution which is particularly frequently used is liquid urea-water solution. A 32.5% liquid urea-water solution is available under the trade name AdBlue®. The method described here is suitable for the operation of a device by which a reducing agent precursor solution can be fed to an exhaust-gas treatment device.
In the development, production and operation of devices for the provision of such liquids for exhaust-gas purification, it must generally be taken into consideration that the (aqueous) liquids can freeze at low temperatures. The 32.5% urea-water solution, for example, freezes at −11° C. In the automotive field, such low temperatures can arise in particular during long standstill periods in winter. When the liquid freezes, a volume expansion typically occurs. This volume expansion can damage the device for delivering the liquid.
For this reason, it is known to evacuate a device of this type upon a stoppage of operation. During the evacuation process, the liquid is removed from the device and replaced with air from the surroundings. Typically, air is drawn into the device via an injector on an exhaust line, while at the same time the liquid is delivered back into a tank. Then, no liquid is present within the device during the standstill phase after a stoppage of operation. Accordingly, it is also not possible for any volume expansion of the liquid to occur within the device. It is, however, a problem that an evacuated device must be refilled before a resumption of operation. During the refilling of the device, there is in particular the problem that undesired leakage of liquid through the injector can occur, in particular if the amount of liquid that must be fed to the device in order to completely refill it is not known exactly. Furthermore, both during the evacuation of the device and during the filling of the device, an unnecessarily long period of operation of a pump should be avoided. A relatively short operating duration of the pump firstly reduces the energy consumption during the filling and evacuation processes. Secondly, it is also possible in this way to prevent damage from being caused to the pump, or increased wear from occurring as a result.